1. Field of the Invention
The present invention relates to bone fixation plate assemblies. More specifically, the present invention relates to a mechanism for preventing the displacement of a screw.
2. Description of Related Art
Spinal fixation has become a common approach in treating spinal disorders, fractures, and for fusion of vertebrate. A common devise used for spinal fixation is a bone fixation plate assembly. Typical bone fixation plate assemblies have a relatively flat, rectangular plate with a plurality of apertures therethrough. Fasteners, such as bone screws, are utilized to secure the bone fixation plate assembly. The screws are firmly tightened to secure the bone fixation plate to the bone or bones to be fixed. There are numerous examples of bone fixation plates existing in the art. These are illustrated in U.S. Pat. No. 5,364,399 to Lowery et al., U.S. Pat. No. 5,601,553 to Trebing et al., U.S. Pat. No. 6,017,345 to Richelsoph, U.S. Pat. No. 6,152,927 to Farris et al., U.S. Pat. No. 6,235,034 B1 to Bray, U.S. Pat. No. 6,139,550 to Michelson, and U.S. Pat. No. 6,258,089 B1 to Campbell et al. The above referenced patents are cited as examples illustrating the general state of the art with regard to bone fixation plate technology, Generally, these types of devices can be utilized for the fixation of any bone or bones, but are more particularly suited for the fixation of the spine with regard to the cervical, lumbar and/or thoracic regions.
The basis of interior fixation or plating is to approach the spine from the anterior or interior-lateral side and use the screws to solidly mount the bone fixation plate to the affected vertebrate. This approach is commonly used in cases of trauma, tumors, and degenerative conditions. Often, in addition to the application of a bone fixation plate, graft material can be combined in an attempt to permanently fuse together adjacent vertebrate. The graft material can consist of bone grafts obtained from other bones in the patient's body or from cadaver bones.
A common problem associated with the use of such bone fixation plates is a tendency of the bone screws to “back out” or pull away from the bone onto which they were fixed. This problem occurs primarily due to the normal motion of the body and the spine. Since the spine is a very dynamic entity and is constantly moving, this problem is especially prevalent in areas of high stress such as the spine. Once the screw becomes loose and pulls away from the bone, the head of the screw can rise above the surface of the bone fixation plate and even work its way completely out of the bone. This creates a number of potentially serious problems given the number and proximity of blood vessels and other critical structures near the locations of spinal fixation plate assemblies.
A number of various designs have been brought forth in attempts to prevent screws from pulling away from the bone and/or to prevent the screws from backing out or pulling away from the surface of the bone fixation plate. For example, the Lowery et al. patent discloses an anterior cervical plating system incorporating a locking screw, which engages the heads of the bone screws used to secure the cervical plate to the vertebrae. The locking screw is positioned above the bone screws and is driven against the heads of the bone screws to rigidly fix the bone screws to the plate. For this locking mechanism to work however, the distance between the heads of the bone screws must be kept to a minimum, thereby limiting the potential applications of the bone fixation plate. Additionally, while the Lowery et al. patent allows for the bone screws to be angled, if the screws are not angled exactly the same amount, which is very difficult to achieve, the locking screw cannot adequately contact both bone screw heads.
Another example of a mechanism for preventing bone fixation screws from backing out or becoming dislodged from the bone is set forth in the Trebing et al. patent discussed above. The Trebing et al. patent discloses a mechanism whereby the bone fixation plate is threaded and is used in combination with a bone screw having both bone engaging threads and a threaded portion near the head of the bone screw which is complimentary to the threaded hole in the bone fixation plate. In this mechanism, the screw is rigidly fixed to the bone fixation plate. It is possible however, to lock the bone screw to the bone fixation plate while leaving a gap between the bone fixation plate and the bone. This problem can cause inferior fixation of the bone or even total failure of the fixation.
Various other mechanisms used to prevent bone screws from pulling out of bones include cams, which engage and lock the screws and the use of expanding head screws, which expand outwardly when adequate force is applied thereto to engage the holes in the bone fixation plate. All of these particular designs have drawbacks including potential for breakage or requiring particular precision and alignment in their application in order to correctly work.
Another apparatus for preventing bone screw back-out from a bone fixation plate is shown in U.S. Pat. No. 5,578,034, issued Nov. 26, 1996, to Estes. The Estes patent discloses a system for internal fixation of bone that includes a bone fixation plate having a number of bores therethrough, a corresponding number of screws each having an enlarged head portion, and an elongated shaft portion defining bone engaging threads thereabout and a non-threaded portion between the head and the threaded portions, and a corresponding number of screw anti-backout members each having a bore therethrough. The screw anti-backout members are inserts positioned within the bores of the fixation plate and are initially sized to slidingly receive an elongated screw shaft therethrough. During application of the fixation plate, the bone screws are advanced through the bone fixation plate bores and the screw anti-backout members which are positioned within the plate bores to screw the bone fixation plate to the underlying bone. Thereafter, the apparatus is sufficiently heated to shrink the bores of the screw anti-backout members, thereby trapping the non-threaded portion of the screw shafts located between the fixation plate and the threaded portions. The anti-backout collars are immobilized within the bore of the fixation plate. The collar and fixation plate remain in fixed relationship to each other after fixation to the underlying bone.
Other types of inserts or collars have been used with bone fixation plates for a variety of reasons such as those shown in U.S. Pat. No. 4,388,921, issued Jun. 21, 1983, to Sutter et al. and U.S. Pat. No. 5,607,428, issued Mar. 4, 1997, to Lin. Sutter et al. discloses a bone fixation plate in which sleeves are placed in openings provided in a bone fixation plate. A screw is placed through the sleeve and into the underlying bone. By tightening the screw, the sleeve is clamped in place with relation to the bone fixation plate thus assuring that the fixation plate will stay rigidly connected with the screws.
The Lin patent discloses a bone fixation plate having a direction adjusting ring disposed in at least one hole in the fixation plate. Upon insertion and tightening of the threaded bone screw, arresting edges of the direction adjusting ring are urged into engagement within the hole to securely fix and retain the direction adjusting ring therein.
Other examples of recent systems that have been built utilize small setscrews or locking screws. Others utilize bulky covers or cams to engage screws to achieve a mechanism for locking and retaining the screw to the plate. One such example is illustrated in U.S. Pat. No. 6,152,927 to Farris et al. The Farris et al. patent discloses a set screw that is longer so that it can remain in the plate and in the bone even if it is backed out a certain distance. The screw can be backed out further so that the bone screws have clearance to enter the holes in the plate. (See FIG. 19). The end result is the screw penetrates through the plate and creates an unnecessary load against the vertebrate, which opposes the bone screws and tends to try and lift the plate off the bone.
U.S. Pat. No. 6,258,089 to Campbell et al. discloses another type of mechanism that prevents the screw from backing out. The Campbell et al. patent discloses the use of a tab that is integral to the plate and machined into the plate therein. The tab must be bent to cover the screw to prevent it from backing out. If a surgeon needs to revise or to fix the screw position, the tab needs to be bent back prior to accessing the screw. As a result, stress is created in the metal and mostly will possibly require the replacement of the plate.
Another example is found in U.S. Pat. No. 6,139,550 to Michelson. The Michelson patent discloses the use of an apparatus thereof for locking three set screws at once utilizing a cam mechanism. In order to guarantee the cam lobes stop exactly where they must be to engage the three screws, the threads in the plate and the threads on the cam must be carefully controlled and timed so that the threads begin exactly the same.
Finally, another example of bone plate and screw guide mechanism is disclosed in U.S. Pat. No. 6,235,034 B1 to Bray. The Bray patent discloses a bone plate including a base plate, wherein a retaining plate is also provided therein. The retaining plate is fixedly attachable to the base plate and the retaining plate covers at least a portion of each of the bone screws. The retaining plate is secured to the base plate with set screws that are inserted into set screw apertures located therethrough. This invention therefore requires a separate screw to retain the bone screws within the base plate and bones.
Accordingly, there is a need for a bone fixation plate assembly that allows positive, rigid fixation of a bone fixation plate to a bone, while allowing movement, stress, or dynamic load sharing of the adjacent vertebrate. More specifically, there is a need for a bone fixation plate assembly that provides a simple reversible locking mechanism built into the plate that prevents the screw from backing out. Additionally, there is a need for a mechanism that maintains and/or prevents the screw from backing out from a bone plate.